For visual comfort outdoors, many people prefer eyeglasses with tinted lenses (commonly called “sunglasses”). For those who wear prescription (“Rx”) eyeglasses, this preference motivates the purchase of at least one additional pair of eyeglasses. Since Rx eyeglasses are relatively expensive, and it is often inconvenient to carry a second pair, an alternative has become popular; namely, the use of an attachable front that ‘clips onto’ the user's clear Rx spectacles (herein “parent” glasses). The attachable front preferably carries tinted lenses with no optical power.
When in use, it is important that jostling does not inadvertently detach the attachment from its parent. And for user convenience when not being worn, it is preferred that the attachment can be stored compactly.
With prior art attachments, the considerations mentioned above cause cosmetic/fit-up issues since, as dispensed, for each style of Rx spectacles the curvature of the eye openings of various pairs can differ markedly from one to another. This variation arises because the three dimensional front curvature (the “base curve”) of a prescription lens depends on its required optical performance, which varies substantially in the populace. Hence each eye opening of a frame into which Rx lenses are being installed must often be ‘reformed’ by the dispenser to enable proper lens positioning. For this reason Rx eyeglasses with prior art attachments usually have an obvious gap between the lenses of the parent and the lenses of the attachment, and hence do not have the appearance of true sunglasses, looking instead like glasses with a double set of lenses. The gap between parent frame and attachment is not only unsightly, but also allows light leakage and troubling internal reflections.
The invented attachment described herein eliminates these deficiencies. In addition, while suitable for converting clear eyeglasses into sunglasses, the invented attachment is not limited to that use. Other applications include, for example, providing on-demand a second (or third) optical power.